Don't know what the famous Grand Central Terminal is? http://grandcentralterminal.com/
There's easy access to there from Penn Station (I'm guessing that's where you're coming from) by taking the subway from Penn Station to Times Square (the 1, 2, or 3 would probably be good, although most, if not all, of the subways in Penn station should go there), and then take the S train to GCT.
Edit: Here's a map of the NYC subway system: http://www.mta.nyc.ny.us/nyct/maps/submap.htm